Low doses of the neonicotinoid insecticide imidacloprid induce ROS triggering neurological and metabolic impairments in Drosophila

Abstract

Declining insect population sizes are provoking grave concern around the world as insects play essential roles in food production and ecosystems. Environmental contamination by intense insecticide usage is consistently proposed as a significant contributor, among other threats. Many studies have demonstrated impacts of low doses of insecticides on insect behavior, but have not elucidated links to insecticidal activity at the molecular and cellular levels. Here, the histological, physiological, and behavioral impacts of imidacloprid are investigated in Drosophila melanogaster, an experimental organism exposed to insecticides in the field. We show that oxidative stress is a key factor in the mode of action of this insecticide at low doses. Imidacloprid produces an enduring flux of Ca2+ into neurons and a rapid increase in levels of reactive oxygen species (ROS) in the larval brain. It affects mitochondrial function, energy levels, the lipid environment, and transcriptomic profiles. Use of RNAi to induce ROS production in the brain recapitulates insecticide-induced phenotypes in the metabolic tissues, indicating that a signal from neurons is responsible. Chronic low level exposures in adults lead to mitochondrial dysfunction, severe damage to glial cells, and impaired vision. The potent antioxidant, N-acetylcysteine amide (NACA), reduces the severity of a number of the imidacloprid-induced phenotypes, indicating a causal role for oxidative stress. Given that other insecticides are known to generate oxidative stress, this research has wider implications. The systemic impairment of several key biological functions, including vision, reported here would reduce the resilience of insects facing other environmental challenges.

Neonicotinoids disrupt memory, circadian behaviour and sleep

“…Indeed, nAChR synaptic signalling is involved in the rhythmic firing of action potentials in clock neurons of Drosophila and other insects 36 , 42 , 63 . Furthermore field relevant concentrations of imidacloprid have also shown to cause degeneration of retinae and reduced visual acuity, providing a further possible route for neonicotinoids to disrupt light input into the clock, and hence the structural and behavioural changes observed here 47 .…”

“…carbachol) evoked Ca 2+ responses in larval neurons. A 2 h exposure of 2.5 µg/L imidacloprid increased gut and brain levels of superoxide reactive oxygen species and decreased mitochondrial function and ATP levels 47 . Chronic exposure of 4 µg/L imidacloprid led to degeneration of the fly retinae and loss of visual acuity, providing further evidence that field relevant sublethal concentrations of neonicotinoids bring about irreversible effects at the neuronal level.…”

Low doses of the organic insecticide spinosad trigger lysosomal defects, ROS driven lipid dysregulation and neurodegeneration in flies

“…But non-pest insects are likely to be exposed to lower doses and the downstream physiological processes that are triggered are poorly understood. In a recent study, low doses of the neonicotinoid imidacloprid were shown to stimulate a constitutive flux of calcium into neurons via the targeted ligand gated ion channels (nicotinic acetylcholine receptors – nAChRs) (Martelli et al, 2020). This causes an elevated level of ROS and oxidative stress which radiates from the brain to other tissues.…”

“…This causes an elevated level of ROS and oxidative stress which radiates from the brain to other tissues. Mitochondrial damage leads to a significant drop in energy levels, neurodegeneration and blindness (Martelli et al, 2020). Evidence of compromised immune function was also presented, supporting other studies (Chmiel et al, 2019).…”

“…The two insecticides differ in their mode of action. Imidacloprid is an agonist causing cation influx into neurons by binding to a site that overlaps with that normally occupied by the native ligand, acetylcholine (ACh) (Buckingham et al, 1997; Martelli et al, 2020; Perry et al, 2008). Spinosad is an allosteric modulator, binding to a site in the C terminal region of the protein (Puinean et al, 2013; Somers et al, 2015).…”